FIG. 1 shows a drilling rig 101 used to drill a borehole 102 into an earth formation 103. Extending downward from the rig 101 is a drill string 104 with a drill bit 105 positioned at the bottom of the drill string 104. The drill string also has a measurement-while-drilling tool 106,and a drill collar 107 disposed above the drill bit 105.
During drilling operations, there are many forces that act on the drill bit 105 and the drill string 104. These forces include weight-on-bit (xe2x80x9cWOBxe2x80x9d) and torque-on-bit (xe2x80x9cTOBxe2x80x9d). The WOB describes the downward force that the drill bit 105 imparts on the bottom of the borehole. The TOB describes the torque applied to the drill bit that causes it to rotate in the borehole. A significant issue during drilling is any bending of the drill string. Bending of the drill string can result from WOB, TOB, or other downhole forces.
The determination of the forces on the drill bit is important because it allows an operator to detect the onset of drilling problems and correct undesirable situations before a failure of any part of the system, such as the drill bit 105 or drill string 104. Some of the problems that can be detected by measuring these downhole forces include motor stall, stuck pipe, and bottom hole assembly (xe2x80x9cBHAxe2x80x9d) tendency. By determining these forces, a drill operator is also able to optimize drilling conditions so a borehole can be drilled in the most economical way.
The typical techniques for measuring the WOB and the TOB at the surface have proven to be unreliable. Forces acting on the drill string 104 between the drill bit 105 and the surface interfere with surface measurements. As a result, techniques and equipment have been developed to measure forces on the drill string near the drill bit. One such method is described in U.S. Pat. No. 5,386,724 issued to Das et al(xe2x80x9cthe Das patentxe2x80x9d), assigned to Schlumberger Technology Corporation.
The Das patent discloses a load cell constructed from a stepped cylinder. Strain gauges are located on the load cell, and the load cell is located in a radial pocket in the drill collar. As the drill collar deforms due to downhole forces, the load cell is also deformed. The strain gauges on the load cell measure the deformation of the load cell, which is related to the deformation of the drill collar.
A strain gauge is a small resistive device that is attached to a material whose deformation is to be measured. The strain gauge is attached in such a way that it deforms along with the material to which it is attached. The electrical resistance of the strain gauge changes as it is deformed. By applying an electrical current to the strain gauge and measuring the differential voltage across it, the resistance, and thus the deformation, of the strain gauge can be measured.
As described in the DAS patent, the load cell may be inserted into the drill collar so that the load cell deforms with the drill collar. The load cell can be constructed of a material that has very little residual stress and is more suitable for strain gauge measurement. Many such materials, may include for example INCONEL X-750, INCONEL 718 or others, known to those having skill in the art.
A BHA is the drill bit and associated sensors and equipment that are located near the bottom of the borehole while drilling. FIG. 2 shows a BHA 200 positioned at the bottom of a borehole 102. The drill bit 105 is disposed at the end of the drill string 104. An MWD tool 106 is disposed proximate to the drill bit 105 on the drill string 104, with a drill collar 107 positioned proximate to the MWD tool 106. FIG. 2 shows two load cells 202, 203 positioned in load cell cavities 205 in the drill collar.
FIGS. 3A and 3B show the load cell 300 disclosed in the Das patent. The load cell 300, as shown in FIG. 3A, has eight strain gauges located on the annular surface 301. The strain gauges include four weight strain gauges 311, 312, 313, and 314, and four torque strain gauges 321, 322, 323, and 324. The weight strain gauges 311-314 are disposed along the vertical and horizontal axis, and the torque strain gauges 321-324 are disposed in between the weight strain gauges 311-314. FIG. 3B shows the load cell 300 disposed in a drill collar 331. When the drill collar 331 is deformed as a result of downhole forces, the load cell 300 disposed in the drill collar is also deformed, allowing the deformation to be measured with the strain gauges.
One aspect of the invention is a load cell comprising a disc member having at least two arcuate apertures and a deformation sensor disposed on a side surface of two of the arcuate apertures. In some embodiments, the disc member includes four arcuate apertures with two deformation sensors disposed in each of two diametrically opposed arcuate apertures.
Another aspect of the invention is a load cell system comprising a load cell with a strain gauge and a load cell circuitry operatively connected to the load cell, the load cell circuitry comprising a non-volatile memory adapted to store load cell calibration data.
Another aspect of the invention is a downhole sensor comprising a drill collar adapted to be disposed around a drill string and a load cell disposed in the drill collar, the load cell comprising a disc member with four radial arcuate apertures spaced at ninety degrees, at least one planar strain gauge disposed on the surface of the disc member, and at least one arcuate strain gauge disposed in each of a pair of diametrically opposed arcuate apertures, the opposed apertures aligned substantially orthogonal to a rotational axis of the drill string.
Yet another aspect of the invention is a method for measuring deformation comprising disposing a deformation sensing element in an arcuate aperture in a load cell, the arcuate aperture adapted to amplify the deformation of the load cell, placing a differential voltage across the deformation sensing element, and measuring a change in an electrical property of the deformation sensing element related the deformation of the arcuate aperture.
Another aspect of the invention includes a load cell comprising a means for amplifying a mechanical deformation of the load cell, and a deformation sensing element disposed on the means for amplifying the mechanical deformation.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.